1. Technical Field of the Invention
The present disclosure relates generally to the treatment of respiratory and cardiovascular conditions, and more particularly, to methods and systems for zero pressure start continuous positive airway pressure (CPAP) therapy.
2. Description of the Related Art
Sleep apnea is a serious medical condition in which patient breathing during sleep pauses abnormally, or is abnormally low. Apnea is categorized as obstructive, central, and combined obstructive and central, though the obstructive sleep apnea (OSA) is the most common. The patient's upper airway repeatedly narrows or collapses, causing pauses in breathing that may extend in duration up to half a minute. Although some degree of apnea is considered normal, in more severe cases, daytime sleepiness and fatigue may result as a consequence of reduced blood oxygen saturation, as well as constant interruptions to sleep cycles resulting from patients gasping for air. There have been studies linking sleep apnea to more severe long-term health issues including heart disease and depression, and recently, to cancer as well. With apnea being strongly linked to obesity, and with obesity being projected to increase, the number of patients suffering from sleep apnea is likely to increase concomitantly.
One common treatment for obstructive sleep apnea is continuous positive airway pressure (CPAP) therapy, where a positive pressure is applied to the patient to prevent its collapse as would otherwise occur during an apnea episode. By retaining the patient's airway, normal, uninterrupted breathing during sleep is ensured. In a basic implementation, CPAP therapy applies a constant pressure that is not tied to the patient's normal breathing cycle. The positive airway pressure is desired in the inspiratory phase when the pressure differences between the lungs and the nose contribute to the collapse of the intermediate airway. Earlier patient breathing assistance devices tended to be uncomfortable to use because of the bulkiness associated with the patient interface, as well as the misapplication of pressure resulting from sub-optimal control methodologies. Various improvements have been developed to reduce discomfort during therapy, particularly at critical points along the patient's respiratory cycle. Thus, what was previously prescribed only for the more severe cases of sleep apnea in which the benefits of treatment outweighed the significant discomfort is now useful for treating a wider spectrum of sleep apnea conditions.
Notwithstanding the increased availability of CPAP devices for home use as a result of these technical improvements, ensuring patient compliance with the prescribed treatment remains a challenge. One complaint is the discomfort associated with the application of pressure against the respiratory efforts of the patient during a waking state. The sense of asphyxiation associated with even a slight pressure and corresponding increase in work of breathing may be distracting enough to a patient such that merely falling asleep becomes difficult. Indeed, positive airway pressure may not be needed until the patient reaches a state of sleep since apnea conditions do not arise until such a state. Thereafter, the application of full therapeutic pressure as properly prescribed in accordance with the patient's condition generally does not interrupt the patient's sleep.
In order to minimize patient discomfort during the pre-sleep state, breathing assistance devices incorporate ramping, where delivered pressure is gradually increased over a set time period. There are a variety of delivery pressure increase curves, also referred to as ramp paths, which may be implemented by the device. These include linear ramp paths, a curved ramp path that increases delivery pressure at a higher rate in the initial time period of the ramp duration, a curved ramp path that increases delivery pressure at a higher rate in the later time period of the ramp duration, and so forth.
With or without ramping, treatment and airflow to the patient begin once the patient wears the interface in existing CPAP devices. Otherwise, exhaled carbon dioxide may accumulate inside the patient interface as well as the conduit between the patient interface and the CPAP device, leading to further discomfort and possible asphyxiation over the repeated inspiration of exhaled breathing gasses.
Again, activating the ventilation source while in the pre-sleep state, even at the low levels of pressure being delivered at the beginning of the ramping, may cause discomfort that prevents the patient from falling asleep. In existing CPAP systems, the patient simply must become accustomed to the slight discomfort at the beginning of treatment. Accordingly, there is a need in the art for a zero pressure start CPAP therapy.